1. Field of the Invention
This invention relates to a monodispersed glycol suspension of fine particles of an inorganic oxide having excellent dispersion stability, and to a method of improving the slipperiness of a polyester film using the monodispersed suspension. More specifically, this invention relates to a monodispersed glycol suspension in which fine particles of an inorganic oxide having a specific shape and a specific particle size distribution are dispersed stably in a glycol solvent without forming agglomerated particles, and a method of improving the slipperiness of a polyester film by using the monodispersed glycol suspension as a raw material for polyester production.
2. Description of the Prior Art
Fine inorganic oxide particles have been extensively used heretofore as a polymer filler and a surface-treating additive. Improvement of the slipperiness of a polymeric film or fibers by including fine inorganic oxide particles is one example of their utility.
Polyester films have been widely used in magnetic tapes, optical photography, capacitors and packaging materials. Various undesirable troubles presumably attributed to the poor slipperiness of polyester films may occur during the process of polyester film production. A polyester film used to make a magnetic tape by coating or vacuum-depositing a magnetic layer on its surface requires especially good slipperiness. If the film slipperines is low, scratches and creases occur on the film surface during film production, coating or depositing of the magnetic layer, or film handling. This results in dropout, and the quality of the resulting magnetic tape is unsatisfactory. Furthermore, the magnetic tape product is required to have good slipperiness because it inevitably needs good travelling property. It has been the practice therefore to form a film from a polyester containing fine particles, thereby providing a roughened film surface and reducing its frictional resistance.
On the other hand, as polyester films have been produced in small thicknesses and magnetic recording has been required to have a high density and high performance in recent years, the polyester films have been required to possess two apparently contradictory properties, slipperiness and surface flatness. As a compromise for these contradictory properties, it has been proposed to form fine and uniform raised and depressed parts on the film surface.
Conventional methods proposed for this purpose include, for example, (1) a method which comprises precipitating a catalyst, a coloration inhibitor, etc. used in polyester synthesis partly or wholly during the polyester-forming reaction thereby to cause them to exist as fine particles in the polymer, and (2) a method which comprises externally adding fine inorganic particles at any desired stage of polyester synthesis.
According to the method (1), the amount or particle diameter of the particles are difficult to control and the formation of coarse particles cannot be avoided because the particles are formed during polyester synthesis.
In the method (2) commercially available materials such as silica, titanium oxide, silica-alumina, silica-magnesia, glass powder, barium sulfate, calcium carbonate, clay, mica, talc, calcium phosphate and magnesium phosphate which have an average particle diameter of 0.001 to 10 micrometers are used according to the end use of the films as the fine inorganic particles to be added (see, for example, Japanese Patent Publication No. 8216/1984 and Japanese Laid-Open Patent Publication No. 3645/1977).
These fine inorganic particles used heretofore have a broad particle size distribution and are mostly irregularly-shaped owing to the methods of their production. For example, the fine particles of silica used are silica having an average primary particle diameter of 0.02 to 0.1 micrometer obtained by thermal decomposition of silicon halides, silica obtained by crushing agglomerated masses having a size of 1 to 5 micrometers obtained from sodium silicate by the wet method, and spherical particles of silica obtained by melting the crushed particles of natural silica. However, they have irregular particle shapes, or have a very broad particle size distribution even when they are nearly spherical, or contain agglomerated particles.
According to the above methods, the slipperiness of the film surface is improved by formation of raised and depressed parts form on it. However, since the particles are non-uniform and the raised and depressed parts lack uniformity, there is a limit to the flattening of the film surface.
As a solution to the above problem of the method (2), the present inventors previously disclosed in Japanese Patent Application No. 48456/1986 that particles produced by hydrolyzing an organometallic compound in an alcoholic solution are fine inorganic oxide particles which are spherical and have a sharp particle size distribution, and by adding these particles to a polyester, an excellent slipperiness improving effect is obtained. When, however, as one embodiment of this method, the fine inorganic oxide particles are added during polyester synthesis, agglomerated particles are sometimes formed. To reduce formation of such agglomerated particles, it is effective to disperse the fine particles highly in a glycol used as a reaction material. However, complex steps such as the addition of a dispersant and ultrasonication are required in order to disintegrate the agglomerated particles. Even by such steps, the inclusion of agglomerated particles cannot be avoided.
The polyester which is an organic polymer intrinsically has no affinity for fine inorganic particles, and when it is formed into a film, voids occur around the fine particles. To avoid this problem, it was proposed to treat the surface of the fine particles with a silane coupling agent. The effect of this procedure is still insufficient.
On the other hand, various methods have been proposed previously for the production of a glycol slurry of fine particles of an inorganic oxide. They include, for example, solvent substitution of an aqueous silica sol produced from water glass by a glycol (for example, Japanese Laid-Open Patent Publication No. 47429/1981), and a method which comprises subjecting an aqueous hydrosol produced from water glass as a raw material to solvent substitution by an organic solvent, heat-treating it in the presence of a monohydric alcohol to bond the alkoxy group chemically to silica, and removing the liquid-phase from the resulting reaction mixture thereby to form powdery silica which can be homogeneously dispersed in an organic solvent (Japanese Laid-Open Patent Publication No. 196717/1982).
The present inventors reproduced the first-mentioned conventional method, and found that the finally obtained glycol suspension contained not only spherical primary particles having a uniform particle size but also many secondary particles formed by agglomeration thereof, and that after filtration and centrifugation, a number of agglomerated particles are still observed.
The present inventors also reproduced the latter method except that a glycol was used instead of the monohydric alcohol, and evaluated the dispersibility of the resulting powdery silica in the glycol. As a result, they observed many agglomerated particles in the final glycol suspension.
C. C. Ballard et al. reported in Journal of Physical Chemistry, 65, 20-25 (1961) that when a silica powder obtained from water glass was treated in ethylene glycol, the glycol could be bonded to the surface of the fine particles. A. K. Van Helden et al. reported in Journal of Colloid and Interface Science, 81 , 354-368 ('81) that spherical silica particles obtained by hydrolyzing ethyl silicate in an alcohol were rendered hydrophobic by heat-treatment in the presence of stearyl alcohol. Since, however, these methods include powderization of fine particles and subsequent heat-treatment in an alcohol, agglomeration of the particles during powderization cannot be avoided. Consequently, they have the disadvantage that after the powder is heat-treated in a solvent (even if the solvent is a glycol), the resulting dispersion contains many agglomerated particles and a monodispersed suspension cannot be obtained.